It is well known that organic polyhydroxy compounds having hydroxyl groups positioned in the cis-form on adjacent carbon atoms or on carbon atoms in a 1,3-relationship react with borates to form five or six member ring complexes. At alkaline pH above about 8.0 these complexes form didiol crosslinked complexes. This leads to a valuable reaction with dissociated borate ions in the presence of polymers having the required hydroxyl groups in a cis-relationship. The reaction is fully reversible with changes in pH. An aqueous solution of the polymer will gel in the presence of borate when the solution is made alkaline, and will liquefy again when the pH is lowered below about 8. If the dry powdered polymer is added to an alkaline borate solution, it will not hydrate and thicken until the pH is dropped below about 8. The critical pH at which gelation occurs is modified by the concentration of dissolved salts. The effect of the dissolved salts is to change the pH at which a sufficient quantity of dissociated borate ions exists in solution to cause gelation. The addition of an alkali metal base such as sodium hydroxide enhances the effect of condensed borates such as borax by converting the borax to the dissociated metaborate.
Known polymers which contain an appreciable content of cis-hydroxyl groups are exemplified by guar gum, locust bean gum, dextrin, polyvinyl alcohol, and derivatives of these polymers. Derivatives tend to react less with borate ions as the amount of substituting groups in the molecule increases. This results because the shear bulk of substituting groups changes the regular, alternating, and single-member branched, linear configuration of the molecule and prevents adjacent chains from approaching as closely as before, and the substitution of secondary cis-hydroxyl positions decreases the number of such unsubstituted positions available for complexing with the borate ion.
Depending on the relative concentration of polymer, and borate anion, the crosslinking reaction may produce useful gels, or may lead to insolubilization, precipitation, or unstable, non-useful gels. The viscosity of the hydrated polymer solution increases with an increase in the concentration of borate anion until a maximum is obtained. Thereafter the viscosity decreases and the gel becomes unstable as evidenced by a lumpy, inhomogeneous appearance and syneresis. As the temperature of the solution increases, the concentration of borate required to maintain the maximum degree of crosslinking, and thus maximum viscosity increases. Derivatization with non-ionic hydroxyalkyl groups greatly improves the compatibility of the polymer with most salts.
Hydraulic fracturing is a widely used method for stimulating petroleum producing subterranean formations and is commonly performed by contacting the formation with a viscous fracturing fluid having particulated solids, widely known as propping agents, suspended therein, applying sufficient pressure to the fracturing fluid to open a fracture in the subterranean formation, and maintaining this pressure while injecting the fracturing fluid into the fracture at a sufficient rate to extend the fracture into the formation. When the pressure is reduced, the propping agent within the fracture prevents the complete closure of the fracture.
The properties that a fracturing fluid should possess are amongst others, low leakoff rate, the ability to carry a propping agent, low pumping friction loss, and easy removal from the formation. Low leakoff rate is the property that permits the fluid to physically open the fracture and one that controls its areal extent. The rate of leakoff to the formation is dependent upon the viscosity and the wall-building properties of the fluid. Viscosity and wall-building properties are controlled by the addition of appropriate additives to the fracturing fluid. The ability of the fluid to suspend the propping agent is controlled by additives. Essentially, this property of the fluid is dependent upon the viscosity and density of the fluid and upon its velocity. Friction reducing additives are added to fracturing fluids to reduce pumping loss due to friction by suppression of turbulence in the fluid. To achieve the maximum benefits from fracturing, the fracturing fluid must be removed from the formation. This is particularly true with very viscous fracturing fluids. Most of such viscous fluids have built-in breaker systems that reduce the viscous gels to low viscosity solutions upon exposure to the temperatures and pressures existing in the formations. When the viscosity is lowered, the fracturing fluid may be readily produced from the formation.
The use of aqueous based fluids to formulate fracturing fluids is well known. Such fluids generally contain a water soluble polymer viscosifier. Sufficient polymer is used to suspend the propping agent, decrease the leakoff rate, and decrease the friction loss of the fracturing fluid. Supplemental additives are generally required to further decrease the leakoff rate, such as hydrocarbons or inert solids, such as silica flour.
Various water soluble polymers have been proposed for use as viscosifiers for aqueous based fracturing fluids, such as polyacrylamides, partially hydrolized polyacrylamides, and various polysaccharide polymers such as guar gum and derivatives thereof, and cellulose derivatives. However, guar gum and guar gum derivatives are the most widely used viscosifiers. Guar gum is suitable for thickening both fresh and salt water, including saturated sodium chloride brines.
It is known to provide concentrated suspensions of borate-containing crosslinking agents for the preparation of crosslinked fracturing fluids. See for example the following U.S. patents: Kinsey, III et al. U.S. Pat. No. 5,488,083; Kinsey, III et al. U.S. Pat. No. 5,565,513; Moorhouse et al., U.S. Pat. No. 6,225,264; and Moorhouse et al. U.S. Pat. No. 6,251,838. It is also known to prepare solutions of borate-containing crosslinking agents. See for example the following U.S. patents: Wadhwa U.S. Pat. No. 4,514,309; Dawson U.S. Pat. No. 5,082,579; Dawson U.S. Pat. No. 5,145,590; and Dawson U.S. Pat. No. 5,160,643.
Mondshine U.S. Pat. No. 4,619,776, incorporated herein by reference, discloses the use of sparingly soluble borates, such as alkaline earth metal borates and alkali metal alkaline earth metal borates for the controlled crosslinking of crosslinkable polymer-containing fracturing fluids. Concentrated suspensions of such borates in hydrocarbon base fluids have been utilized for the crosslinking of fracturing fluids containing guar gum or derivatives thereof, particularly hydroxypropyl guar, and have achieved commercial success. These concentrates contain an organophilic clay suspending agent to keep the borate crosslinking agent suspended therein, thus preventing settling thereof.
Concentrated aqueous suspensions of the sparingly soluble, alkaline earth or alkali metal alkaline earth metal borates having also been commercially successful. Such aqueous concentrates contain a high concentration of the sparingly soluble borate, a suspending agent, and a suspension stabilizer. A typical problem with such aqueous suspensions in fracturing operations is the variation in the time to crosslink (i.e., the crosslink time), the borate crosslinkable polymer containing fluid to produce the fracturing fluid. Thus the sparingly soluble borate particles slowly dissolve until the water eventually reaches saturation with respect to the borate salt. As the boron (borate) level in the aqueous phase increases, faster and variable crosslink times occur. Thus the degree of this variation depends upon the time and temperature of mixing and the static (aging) time of the borate suspension. These differences require the use of additives and chemicals and related costs in fracturing operations to modify the rate at which the crosslinked polymer structure forms.
Accordingly, an aqueous borate concentrate that provides a controllable crosslink time will improve over the currently available aqueous concentrate, and over the various prior art borate concentrates and solutions which contain non-aqueous solvents.